SKN1 AND KRE6 DEFINE A PAIR OF FUNCTIONAL HOMOLOGS ENCODING PUTATIVE MEMBRANE-PROTEINS INVOLVED IN BETA-GLUCAN SYNTHESIS

KRE6 encodes a predicted type II membrane protein which, when disrupte d, results in a slowly growing, killer toxin-resistant mutant possessi ng half the normal level of a structurally wild-type cell wall (1-->6) -beta-glucan (T. Roemer and H. Bussey, Proc. Natl. Acad. Sci. USA 88:1 1295-11299, 1991). The mutant phenotype and structure of the KRE6 gene product, Kre6p, suggest that it may be a beta-glucan synthase compone nt, implying that (1-->6)-beta-glucan synthesis in Saccharomyces cerev isiae is functionally redundant. To examine this possibility, we scree ned a multicopy genomic library for suppression of both the slow-growt h and killer resistance phenotypes of a kre6 mutant and identified SKN 1, which encodes a protein sharing 66% overall identity to Kre6p. SKN1 suppresses kre6 null alleles in a dose-dependent manner, though disru ption of the SKN1 locus has no effect on killer sensitivity, growth, o r (1-->6)-beta-glucan levels. skn1 kre6 double disruptants, however, s howed a dramatic reduction in both (1-->6)-beta-glucan levels and grow th rate compared with either single disruptant. Moreover, the residual (1-->6)-beta-glucan polymer in skn1 kre6 double mutants is smaller in size and altered in structure. Since single disruptions of these gene s lead to structurally wild-type (1-->6)-beta-glucan polymers, Kre6p a nd Skn1p appear to function independently, possibly in parallel, in (1 -->6)-beta-glucan biosynthesis.